The field is lumber drying and handling. More particularly, the present invention relates to a vertical-progressive, convective drying process for lumber. In lumber manufacture, green lumber is produced from logs of fixed or multiple lengths. Lumber is then stacked on cars in which each layer is separated by stickers. Cars of lumber are queued for horizontal movement into the dry kilns in a batch process.
Conventional lumber drying systems presently used in lumber mills are typically batch, convective dry kilns. The green lumber, having a green moisture content of anywhere from 30 to 250%, is stacked on individual cars with stickers to separate courses or layers of lumber. These cars are moved into the dry kilns, the doors closed, and exposed to a drying schedule. Hot air is moved horizontally across the lumber, transferring heat to the boards and providing for removal of the moisture. Typically, schedules are used to dry in the shortest possible time with minimal degrade losses from warp. The final moisture content target is consistent with grading or customer requirements, which for softwoods is about 10 to 20% moisture content.
Because of the variability in moisture content and growth properties of green lumber, and the stress development and differential shrinkage that occurs during the typical drying process, many of the above-mentioned desirable characteristics are not consistently obtained. For example, conventionally dried lumber usually has a range of moisture content within the load after drying and a certain percentage of the lumber must be either redried to reduce the moisture content to the required level or sold at a lower grade. In addition, as moisture content is lowered, warpage occurs which reduces the lumber value. This warpage also interferes with further processing steps, such as planing, where machine downtime from lumber breaks can be excessive. Much of the volume of excessively wet lumber or overdried lumber is caused by poor endpoint control in drying. Present state-of-the-art time schedules to establish the drying endpoint. It is well known that a typical means of reducing wet lumber is to overdry it, increasing the amount of warp.
In existing dry kilns, a considerable amount of energy is required per unit volume of dried lumber, and represents a substantial portion of the energy required in a lumber mill. A significant amount of the drying energy is wasted because of the inability to economically recover energy from exhaust air. It is also a recognized problem that stacking and baffling of lumber loads causes inefficient airflow of the heated air leading to longer drying times and poor moisture content uniformity. Several types of drying systems have been developed to overcome the above deficiencies. For example, restraint systems, such as dead loads of concrete, have been used to reduce warpage during drying. While this is effective, it is costly to implement, displaces kiln volume, can cause safety problems, and increases energy losses.
Progressive kilns have been developed to reduce energy and labor costs. However, these kilns are not efficient in air movement and may require excessively long drying times. A vertical continuous kiln has been designed which eliminates the need for restraints in that the lumber is self-restrained and increases energy efficiency in the airflow pattern. As an example of such a kiln and to supplement the disclosure herein, the U.S. patent to Northway et al--U.S. Pat. No.4,261,110 issued Apr. 14, 1981--is incorporated herein by reference. Unfortunately, the complex lumber course removal mechanism disclosed in the patent requires nearly perfect stacking to prevent jamming of the mechanism and board damage. It also uses individual stickers which increase handling and stacking problems. In addition, the air reversal points are not well separated, which will cause excessive air leakage from one zone to another and subsequent poor moisture content uniformity. The continuous dryer is not well designed for endpoint moisture content measurement and determination since dried lumber exits only in single or several courses at a given time.